According to generally accepted national codes, what is the smallest size pipe that can be used to carry condensate from the secondary drain pan?

The correct choice is based on the standards set by national codes for the proper sizing of pipes used to carry condensate from secondary drain pans. Using a 3/4" nominal pipe is generally accepted because it provides adequate capacity to handle the expected volume of condensate in typical HVAC systems.

A larger diameter allows for better flow, reducing the risk of clogs or backups that could occur if the pipe were too small. This size helps ensure that condensate can be evacuated effectively, preventing potential water damage and maintaining optimal performance of the air conditioning system.

While smaller sizes such as 1/2" or 3/8" nominal may not suffice for the expected condensate load, and larger sizes like 1" nominal, while functional, can be unnecessarily oversized for typical installations, 3/4" represents a balance of efficiency and effectiveness per current codes.